Chop parsnips, carrots, celery root, rutabaga, and yellow onion into 1-inch pieces and place in a large sealable container. Add garlic, rosemary, olive oil, salt, and pepper; seal container and shake well to coat vegetables evenly. Distribute vegetables in a single layer in two 9x13-inch baking dishes; scrape remaining oil and seasonings from container onto vegetables.
Roast in preheated oven until browned and easily pierced with a fork, about 45 minutes.
